DESCRIPTION: An understanding of the relationship between functional ability and physiological characteristics among the elderly is key to future planning of the need for health services. One physiological measurement commonly available in national surveys and known to be correlated at younger ages with increased disability and mortality is the (BMI). High body mass indices have been associated with increased risk of a variety of diseases and functional limitations. At the same time, there is evidence that the average body size in the population is increasing -- a greater proportion of the population at all ages is now classified as obese. This project will extend the current understanding of the relationship between body size and functional limitation by using two nationally representative, longitudinal data sets of individuals at very old ages; the Longitudinal Study of Aging (LSOA) and the Asset and Health Dynamics of the Oldest-Old (AHEAD). The project will examine questions about the current relationship between body size and disability at old ages -- are people at the extremes of body mass more disabled? to what extent and in what ways? The work will also investigate how socioeconomic characteristics, health behaviors, and disease interact with body size and functional limitation -- is body mass an intermediate variable in the disablement process, or does it act directly? In addition, the investigators examine the effect of body mass on the progression of disability -- is the rate of functional decline with age faster among those with high BMI compared to others? how significant are changes in body mass for predicting functional limitation? Finally, the project will estimate the extent to which extreme body size affects active life expectancy.